I use a stockman style for my drainpipe, same concept as a durso but a bit more compact. If the air-hole(siphon break) is too small the water level will rise and fall in the overflow. My friends tank had the intake too high in the overflow, caused a whirlpool that happily sucked in air and made that nice toilet flushing sound... constantly. Are you using a durso now? I see something about an elbow but not sure how it is setup.